I am like 90% sure there is a fundamental incompatibility with the new Garageband and Amplitube. When I upgraded garageband I tried to use the amplitube models and couldn't, I contacted Amplitube customer support and unfortunately I can't remember the exact reason but it is something like the new Garageband is 64 bit and the Amplitube is 32 bit and won't even see it. You can install the older version of Garageband and use Amplitube but then you don't have access to the drummers so yer screwed. I suppose on the upside the new Garageband bass amp models are better. I just accept that the money I spent on the Amplitube SVT model and Cab is lost and I've moved on.
If I'm wrong about this, and I hope I am, please tell me.
Here's the fix from IK Multimedia Tech Support:
We are working with Apple on a permanent fix for this issue.
Please note the below is only a workaround, not the permanent solution. There are some issues still present such as the inability to try out Custom Shop gear in GarageBand 10 and an issue with AmpliTube 3 where the Preset Browser cannot be displayed, instead you receive an error. Presets can still be browsed by using the drop-down menu in the upper left corner instead.
1. Authorize AmpliTube 3 using the Authorization Manager application
2. Launch AmpliTube 3 in standalone from your Applications folder and verify the software is authorized
3. Then, open a Finder window and navigate to the below location:
(Note that the User Library is hidden by default, to view this either press the 'alt' key while viewing the Finder > Go menu, or, open a Finder window and click up top on View > Show View Options. There is an option 'Show Library Folder', enable this.)
Mac HD > Users > username > Library > Preferences >
Locate the file named 'com.ikmultimedia.AmpliTube 3.plist'.
4. Copy this file over to the following folder location:
Mac HD > Users > username > Library > Containers > com.apple.garageband10 > Data > Library > Preferences
There may already be a file of the same name in this location, select to Replace the file.
Once the file has copied over, please launch GarageBand 10. Insert AmpliTube 3, you are now authorized and ready to work!
Please note, due to this workaround, when you purchase new gear models in the Custom Shop, you will need to repeat the process with the updated .plist file after verifying the new gear is appearing in standalone mode.